Statistics of Ion Downward Flows in the Ionosphere during Subauroral Polarization Streams
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摘要:
亚极光区极化流是地球磁层–电离层耦合过程中最重要的现象之一,对电离层具有重要的调制作用。多数情况下,亚极光区极化流发生时会伴随局地离子上行流动,目前已对其有充分的研究,但对离子下行的研究却甚少。本文分析了DMSP卫星在2001 - 2015年间16个地磁暴中的观测数据,在483个亚极光区极化流事件中找到102个离子下行事件,并对其开展统计研究,分析离子下行在亚极光区极化流事件中的发生率,在南北半球的分布以及有助于离子下行形成的条件。结果表明:离子下行在亚极光区极化流事件中发生概率较小,离子下行事件的发生与分布具有比较明显的南北半球差异,并且亚极光区极化流的速度与离子下行速度具有高度相关性,表现为线性关系;而离子下行极有可能是因为双极扩散与地球重力共同作用而产生的,并且离子的热运动强弱对离子下行的速度大小也具有重要影响。这些研究结果有助于进一步理解亚极光区极化流对地球电离层动力学的调制作用。
Abstract:During geomagnetically disturbed times, enhanced westward plasma flows appear in the duskside subauroral ionosphere and are termed as Subauroral Polarization Streams (SAPS). SAPS are usually believed as one of the most important features of the magnetosphere-ionosphere coupling and play an important role in regulating the ionospheric dynamics. Generally, SAPS can give rise to local ion upward flows, but there are few studies on ion downward flows. Based on the observations of DMSP satellites during 16 magnetic storm from 2001 to 2015, we find 102 cases with ion downward flows in all the 483 SAPS events. The occurrence rate of ion downward flows, the distribution of ion downward flows in the Northern and Southern Hemispheres, and the favored conditions for the formation of ion downward flows are further analyzed in these SAPS events. The results show that the occurrence rate of ion downward flow is relatively low in SAPS events, and the number and distribution of ion downward flow events are different on the two hemispheres. A linear relationship is found between the SAPS and ion downward flow velocities. Furthermore, the formation of ion downward flow is most probably caused by the combined effects of ambipolar diffusion and gravity. The intensity of ion thermal movement has significant influence on the velocity of ion downward flow. These results will help us to further understand the regulating effects of SAPS on the ionosphere.
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图 1 DMSP F17在南半球观测到的一个SAPS发生时离子下行典型事例。图中从上到下分别为离子水平方向速度、离子垂直方向速度、电子能量通量、离子能量通量、电子数密度以及离子温度
Figure 1. A typical case of ion downward flow observed by DMSP F17 during a SAPS event in the Southern Hemisphere. From top to bottom are the ion horizontal drift velocity, ion vertical drift velocity, electron energy flux, ion energy flux, electron number density and ion temperature
图 2 2013年3月17日磁暴时DMSP F16~F18观测到的SAPS事件以及相应的离子上行与下行的统计情况。(a) 该磁暴所有SAPS事件中离子上行与下行的数量,(b) 离子下行事件在南北半球的分布
Figure 2. Statistical characteristics of ion upward flow and downward flow with SAPS events from DMSP F16~F18 observations during the 17 March 2013 magnetic storm. (a) Number of ion upward flow and downward flow cases with SAPS events. (b) Distribution of ion downward flow events on the Northern Hemisphere (NH) and Southern Hemisphere (SH), respectively
图 3 2001-2015年间共16个地磁暴DMSP F12~F18观测到的SAPS事件以及离子下行的统计情况。 (a) 不同等级磁暴中发生的SAPS事件以及离子下行的数量,(b) 南北半球发生的SAPS事件数量。(c)~(f)地球磁层平静(Kp≤4)和扰动(Kp>4)时期所有离子下行事件在南北半球的分布
Figure 3. Statistical characteristics of ion downward flow with SAPS events from DMSP F12-F18 observations during 16 magnetic storm of 2001-2015. (a) The number of all ion downward flow cases with SAPS events on the different storm level, (b) the number of SAPS events on the Northern and Southern hemispheres. (c)~(f) Distribution of ion downward flow events on the Northern and Southern hemispheres during the quiet (Kp≤4) and disturbed (Kp>4) times, respectively
表 1 2001-2015年间发生的16个地磁暴
Table 1. 16 geomagnetic storms during 2001-2015
Date Storm level Dstmin/nT Date Storm level Dstmin/nT 2001.03.31-04.02 Severe –437 2005.09.15-09.17 Moderate –70 2001.09.30-10.02 Strong –143 2010.04.05-04.08 Moderate –90 2002.04.17-04.19 Strong –151 2010.08.03-08.05 Moderate –81 2003.11.20-11.22 Severe –490 2012.06.16-06.19 Moderate –69 2004.04.01-04.07 Strong –149 2013.03.17-03.19 Strong –132 2004.11.07-11.09 Severe –394 2013.05.31-06.03 Strong –137 2005.01.21-01.23 Moderate –92 2013.06.27-07.02 Strong –111 2005.02.05-02.11 Moderate –68 2015.03.17-03.19 Strong –223 -
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